1. Field of the Invention
The present invention relates to a semiconductor memory device comprising a ferroelectric capacitor and a method of manufacturing this semiconductor memory device.
2. Description of the Related Art
Nonvolatile memories (hereinafter referred to as FeRAMs: Ferroelectric Random Access Memories) having a capacitor portion composed of a ferroelectric substance can be used for high speed operations without any batteries. Accordingly, the FeRAM is now often mounted in a non-contact card such as an RF-ID by being mixed with a logic circuit or the like. The FeRAM is also expected to be a memory that replaces conventional SRAMs (Static Random Access Memories), flash memories, and DRAMs (Dynamic Random Access Memories).
The conventional FeRAM is formed using, for example, the process described below. First, a transistor is formed on a silicon substrate. Then, a first interlayer insulating film is deposited on the transistor. The first interlayer insulating film is then flattened. Then, a ferroelectric capacitor is formed on the first interlayer insulating film. Subsequently, annealing is carried out to recover the ferroelectric capacitor from possible damage. Then, a hydrogen barrier film is deposited on the ferroelectric capacitor. Then, a second interlayer insulating film is deposited on the hydrogen barrier film. The second interlayer insulating film is then flattened. Then, contact holes are formed through the second interlayer insulating film and the hydrogen barrier film. Subsequently, annealing is carried out to recover the ferroelectric capacitor from possible damage. Then, a metal material is filled into the contact holes to form contacts. Subsequently, interlayer insulating films or multilayer interconnects are formed as required.
Such a conventional FeRAM has the problems described below. The ferroelectric characteristic of the ferroelectric capacitor is known to be degraded in a hydrogen atmosphere. Thus, to prevent the capacitor from being degraded in the hydrogen atmosphere during an interconnection step after the capacitor has been formed, a hydrogen barrier film is normally formed on the capacitor. However, the hydrogen barrier film inhibits hydrogen from diffusing from a surface of a wafer to the interior of the capacitor. Consequently, the hydrogen barrier film may inhibit the diffusion of water molecules, which have a larger molecular weight than hydrogen. That is, the hydrogen barrier film may produce a secondary effect of suppressing the release of gases from under the hydrogen barrier film. As a result, when annealing is carried out to recover the capacitor from possible damage done to the capacitor during processing, volatile components such as moistures and gases may be generated from the interlayer insulating film. The volatile components are deposited immediately below the hydrogen barrier film at a high concentration owing to the secondary effect.
Under these circumstances, when contact holes are formed through the hydrogen barrier film, the volatile components accumulated through the contact holes are explosively diffused. The explosive release of gases may cause voids to be formed in the contact holes when a metal material is filled into the contact holes to form contacts. Disadvantageously, this may prevent the contacts from being appropriately filled, thus, for example, degrading the reliability of interconnects.